Sewing machine drive motor control system

ABSTRACT

A speed-control circuit for a motor of the sewing machine is disclosed. The circuit includes a controller, thyristors for controlling the rotation of the motor in dependence upon the operation of the controller, operational amplifiers activated in dependence upon the operation of the controller to produce a speed designating voltage, a frequency - voltage converter which receives a frequency representing the speed of rotation of the main shaft of the sewing machine, a comparator for comparing that speed designating voltage with the voltage of the frequency-voltage converter to detect a motor drive phase, and a light emitting diode and a phototransistor activated by the motor drive phase and operative for controlling the ignition phase of those thyristors.

This is a division of application Ser. No. 28,185, filed Apr. 9, 1979, now U.S. Pat. No. 4,323,833.

BACKGROUND OF THE INVENTION

The invention relates to a control for a drive motor in a sewing machine. More particularly, this invention relates to a control system which will regulate motor speed in a stable fashion as such speed is increased and decreased and which will also stop the motor.

A prior art system of this kind is shown in FIG. 1, in which together with thyristors 2 diodes 3 form a full-wave rectifying bridge which supplies a DC motor. Thyristors 2 are phase-controlled by a gate control device (not shown) connected to their gates G₂, and the motor 4 is thus speed-controlled by the phase-controlled voltages applied to gates G₂, and numerals 7 and 8 are free wheel diodes. A braking thyristor 9 is connected in parallel with the armature 5, and after receiving a control signal (not shown) at its gate G₉, short-circuits an armature 5 during motor braking, so as to consume the motor's rotational energy by dissipating it in the armature's own resistance. FIG. 2 shows the operating relations of the circuit elements during the drive and braking periods of the motor. In FIG. 2 line G₂ designates a presence or non-presence of a gate signal of motor control thyristors 2, and during the period t₁ to t₂ indicates a phase-control voltage applied to thyristors 2. Line G₉ shows a gate signal applied to braking thyristor 9, and the period between t₂ and t₄ shows a period of time in which the thyristor 9 is conductive (as shown by the level of line 9) to brake motor 4. Line N shows the speed of the motor 4, which during the period between t₁ and t₂ is driven at a speed determined by the gate signal G₂. When thyristor 9 is conductive at the point t₂ for braking the motor, field coil 6 is energized via short-circuited thyristor 9, since the thyristor 2 has been fired. Armature 5, therefore, generates reverse EMF from time t₂ and the armature 5 carriers current via thyristor 9. The rotational energy of the armature and the elements connected thereto is then consumed by the internal resistance of armature 5 and motor 4 is rapidly stopped. In FIG. 2, motor speed N is reduced at time t₂, following the solid curved line. But, depending upon AC power source phase, speed, and motor load at time t₂, the required stopping time for the motor can vary as is shown by dotted lines a and b. Therefore in order to exactly stop the motor in a predetermined period, gate signals G₂ and G₉ are prolonged respectively, to times t₃ and t₄. Accordingly, the field coil 6 is unnecessarily heated by a comparatively large electric current until time t₃ --i.e. even after the motor stops. The prolongation of gate signal G₉ serves the purpose of preventing the motor from rerotating. Such rerotation otherwise take place because cutoff of thyristor 2 is unstably dependent upon the phase of the AC power source at time t₃, and may lag about a half cycle behind time t₃, causing field coil 6 to be energized after cutoff of thyristor 9. This is the reason why such a timing control is necessary. However, such a timing control may cause erroneous operations of the circuit elements.

The present invention has been provided to eliminate such defects and disadvantages of the prior art.

SUMMARY OF THE INVENTION

It is a basic object of the invention to compensate for variations of the motor load and the power source over the whole speed range of the motor to thereby provide special stabilization.

It is another object of the invention to avoid supplying electric current to the motor while simultaneously generating electricity therein, to thereby stop the motor quickly and accurately.

Many other features and advantages of the invention will be apparent from the following description of its actual embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1 is a prior-art control circuit;

FIG. 2 are graphs associated with FIG. 1;

FIG. 3 is a schematic diagram of the basic parts of invention;

FIG. 4 are graphs associated with FIG. 3;

FIGS. 5A and 5B represent circuit showing an actual embodiment of the invention, including the circuit of FIG. 3;

FIG. 6 are graphs associated with FIGS. 5A and 5B;

FIG. 7 is a motor speed detecting circuit used in combination with the circuit in FIGS. 5A and 5B;

FIG. 8 is a schematic perspective drawing of a sewing machine embodying the invention;

FIG. 9 are graphs associated with the circuit in FIG. 7;

FIGS. 10 to 12 are further graphs associated with the circuit in FIGS. 5A and 5B;

FIG. 13 is a second embodiment of the invention;

FIG. 14 are graphs associated with the embodiment in FIG. 13; and

FIG. 15 are further graphs associated with the embodiment of the invention shown in FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 3, thyristors 10 and diodes 3 constitute a full wave bridge rectifier. G₁₀ designates gate terminals of thyristors 10, to which gates a motor control signal can be applied. Reference numeral 11 indicates a DC motor having an armature 5 to which a field coil 6 is connected via diode 12 which conducts current in the same direction do thyristors 10. Braking thyristor 13 conducts current in the same direction as do thyristors 10 and is connected in parallel with a series network of armature 5 and diode 12. G₁₃ designates a gate terminal of thyristor 13. A second braking diode 14 conducts in an opposite direction to thyristors 10 and is connected in parallel with a series network of field coil 6 and diode 12 to serve as a freewheel diode for field coil 6.

FIG. 4 differs FIG. 2 in that braking thyristor 13 receives a gate pulse G₁₃ at time t₅ having a width which is considerably narrower than the braking period. Just after thyristor 13 is fired the motor is braked and thyristor 13 cuts off when motor speed N is reduced to zero at time t₇. However, depending upon loading conditions of motor 11, the conductive period of the thyristor 13 is extended (as shown by the dotted line) until the speed N of motor 11 ceases at time t'₇. This will be explained with reference to FIG. 3. While thyristors 10 are ignited by gate signal G₁₀ and motor 11 is rotated, braking signal G₁₃ and maximum signal G'₁₀ are delivered to braking thyristor 13 and thyristors 10 at time t₅, for braking motor 11. In FIG. 4, signal G'₁₀ is shown at a level higher than signal G₁₀ for the sake of convenience to show that thyristors 10 are fired for the maximum ignition angle. Such an ignition angle temporarily increases current flowing through field coil 6 to make the motor brake more effective. Braking signal G₁₃ also fires thyristor 13 for a maximum ignition angle for the same purpose. Field coil 6 is thus strongly energized by the maximum ignition of thyristors 10 and 13, and, on the other hand, armature 5 is not supplied with current because it is short-circuited by thyristor 13. When each of the ignition signals disappears at time t₆, thyristor 10 is cut off at a zero crossover point of AC power source 1. Meanwhile, thyristor 13 continues to discharge the electromagnetic energy stored in field coil 6 and simultaneously consumes current generated in armature 5, via a short circuit path through diode 14 across field coil 6, just after cutoff of thyristors 10. Thus, rotating energy of motor 11 is consumed by resistance of armature 5 and field coil 6, and the motor 11 is dynamically braked and is rapidly stopped. Thyristor 13 does not receive a braking signal G₁₃ during the braking period, but the current generated during motor rotation holds the thyristor conductive until it disappears. A diode 12 passes this current through field coil 6, and prevents this current from passing through only thyristor 13 and thus directly short-circuited.

FIGS. 5A and 5B show an embodiment of a control circuit embodying the gate circuits which were omitted in FIG. 3. Motor 11 is used for driving a sewing machine having a speed control and a brake. Thyristors 10 are fired by a phototransistor PTr₁ receiving light from light-emitting diode LED₁, which is driven by an output of operational amplifier OP₄, further described below. In this control circuit, R₁, R₂ and R₃ are resistors. D₁ is a diode. ZD₁ is a Zener diode for compensating voltage variations of power source 1. T is a stepdown transformer. Diodes D₂ constitute a full wave rectifier. D₃ is a diode. C₁ is a voltage smoothing capacitor. R₄ is a resistor. ZD₂ is a Zener diode compensating for voltage variations of the power source 1 to provide a constant voltage between connections A and B. OP₁ is an operational amplifier having an inverting input receiving a constant voltage divided by resistors R₅ and R₆ between connections A and B, and having a non-inverting input receiving a full wave rectified voltage from diodes D₂, after division by resistors R₇ and R₈. The value of resistor 8 is set much larger than those of resistors R₅, R₆, and R₇ which are of like value. Values of resistors R₅ -R₈ are so set that the power source period O-π may be bigger than the (+) and (-) sides of operational amplifier OP₁. ZD₃ is a Zener diode for protecting operational amplifier OP₁. OP₂ is an operational amplifier having a non-inverting input receiving a divided voltage between the connections A and B from a potentiometer R₉ adjusted by a machine control (not shown). Potentiometer R₉ has a wiper CONT usually returned to its lowest end. As the wiper is moved up, the rotation speed of the motor is increased, and the motor is stopped when the contact is moved all the way down. The output side operational amplifier OP₁ is connected to connection A via resistor R₁₀, and is connected to connection B via a small resistor R₁₁ and a capacitor C₂. The output of operational amplifier OP₂ is connected to its inverting input and is also connected to a point between the resistor R₁₁ and the capacitor C₂, via resistor R₁₂ and diode D₇. These operational amplifiers OP₁ and OP₂ and capacitor C₂ operate signals J₁ -J₄ (shown in FIG. 6) at the output of operational amplifier OP₁. FIG. 6 indicates the speed of motor 11 in dependence upon position of the wiper CONT. The output of operational amplifier OP₁ is connected to the non-inverting input of operational amplifier OP₄. As is shown in FIG. 6, operational amplifier OP₁ provides an output signal at time t₁ just after power source 1 starts a cycle at time O. Capacitor C₂ then starts to charge via resistor R₁₀, and cuts off the output of operational amplifier OP₁ at a point t₂, just before the source begins a new cycle. Capacitor C₂ then rapidly discharges via the resistor R₁₁ and the operational amplifier OP₁. Operational amplifier OP₂ has an increasing voltage at its non-inverting input as wiper CONT shifts upwardly to thereby increase its output and thereby increase the charging speed of the capacitor C₂. As a result, the charging potential is increased to enlarge the triangular signal from voltage J₁ to voltage J₄. Voltage J₁ corresponds to the lowest position of the wiper CONT sewing basting stitches slowly. OP₃ is an operational amplifier having an inverting input which receives a voltage divided by resistors R₁₃ and R₁₄ between the connections A and B, having a non-inverting input connected to the wiper CONT, and having an output terminal providing a low-level signal when wiper CONT is at its lowest position and providing a high-level signal when the wiper is at positions other than this lowest position. Thus, the operational amplifier OP₃ checks the moving and stopping conditions of motor 11. An output signal C from operational amplifier OP₄ lights a light-emitting diode LED via resistor R₁₅ and operates the phototransistor PTr₁ to control the phase of thyristors 10 in dependence upon the lighting phase of the phototransistor.

The circuit in FIG. 7 is used in association with the circuit in FIGS. 5A and 5B and includes a phototransistor PTr₂. Phototransistor PTr₂ is operated by a rotation speed detector on the main shaft of the sewing machine, and receives light pulses with a period which is proportional to the rotation speed of the sewing machine.

In FIG. 8, reference numeral 15 is an upper shaft of the sewing machine and 16 is a photointerrupter fixed to the sewing machine, including the phototransistor PTr₂. A numeral 17 is a screen disc having a plurality of slits 18, and the phototransistor PTr₂ is illuminated each time one of the slits 18 is opposite thereto.

In FIG. 7, the reference letter A indicates a connection to connection A in FIG. 5A and 5B. OP₆ is an operational amplifier. R₁₆, R₁₇, and R₁₈ are resistors and a terminal B is connected to terminal B in FIG. 5B, constituting a Schmitt circuit together with phototransistor PTr₂. As shown in FIG. 9, the emitter voltage D of phototransistor PTr₂ has a substantially rectangular waveform relating to the width of the slits 18 in screen disc 17. Advancing to the right side, the width and the period of voltage D become smaller, because wiper CONT in FIGS. 5A, 5B is progressively moved upwardly and the speed of motor 11 increases. The voltage E across capacitor C₃ will be respectively, the differential values of the rise and fall of voltage D. OP₇ is an operational amplifier. R₁₉ and R₂₀ are resistors, C₄ is a capacitor, which together with capacitor C₃ form a frequency-voltage converter. The voltage E is integrated as is shown in FIG. 9 to form a speed signal which is proportional to motor speed. Each example of those values is shown as F₁ to F₄ in FIG. 6. Terminal F in FIG. 7 is connected to a terminal F in FIGS. 5A and 5B.

A reference numeral 19 in FIG. 8 is a part of photointerrupter 16 which includes phototransistor PTr₃ in FIGS. 5A and 5B. Numeral 20 is a screen element having a large diameter through an angle of 180° and having a small diameter through an angle 180° . FIG. 10 shows the operation of the phototransistor PTr₃ versus the rotation angle of the upper shaft 15 of the sewing machine which latter is represented on a horizontal axis. As shown in FIG. 10, phototransistor PTr₃ becomes conductive just before the upper shaft 15 completes a half rotation to bring the needle to its upper dead point, and becomes nonconductive just before the upper shaft completes another half rotation to bring the needle to its lower dead point. Referring to FIGS. 5A and 5B, the collector of phototransistor PTr₃ is connected to point A, and its emitter is connected through the inverter IN₁ to an input of an exclusive OR circuit ExOR and is also connected to another input of the ExOR through a delay circuit composed of the resistor R₂₁ and the capacitor C₅. R₂₂ is a resistor. The output of ExOR provides rising and falling pulses of a width equal to the time delay of the delay circuit, as shown in FIG. 10, at the upper and lower dead points of the needle. Such pulses are applied to AND circuits AND₁ and AND₂ for stopping the needle at the lower and upper dead points. The output of inverter IN₁ is applied as a second input to the AND circuit AND₁ and is also applied to another input of AND circuit AND₂ through an inverter IN₂. SW_(B) is a switch for producing basting stitches. If switch SW_(B) is closed, the sewing machine is driven at a low speed and it is possible to stitch intermittently by stopping the needle at the upper dead point on each of the sewing machine. When switch SW_(B) is opened, one of the inputs of NAND circuits NA₁ and NA₂ becomes logically high, and becomes logically low when switch SW_(B) is closed. SW_(C) is a switch for stopping the needle. When switch SW_(C) is opened, the needle is stopped at its lower dead point and when switch SW_(C) is closed, the needle is stopped at its upper dead point. When switch SW_(C) is opened, the other input of NAND circuit NA₁ becomes logically high, and when the switch is closed that input becomes logically low. R₂₃ are pull-up resistors attached to point A. The output of NAND circuit NA₁ is applied as a third input to AND circuit AND₂ and is also applied as a third input to AND circuit AND₁ via an inverter IN₃. Basting stitch switch SW_(B) is connected to the base of the transistor Tr₁ via inverter IN₄ and resistor R₂₄. The collector of transistor Tr₂ is connected to the connection between resistor R₁₂ and diode D₇ in order to ground the output of operational amplifier OP₂ when switch SW_(B) is closed. In this case, resistors R₁₀ and R₁₁ are so chosen that the output of the operational amplifier OP₁ may form signal J₁ in FIG. 6. Output Y of operational amplifier OP₃ is connected to one input of AND circuit AND.sub. 3 for detecting the rising of a signal Y on initial operation of the controller, and is applied to the other input of AND circuit AND₃ through a delay circuit including inverter IN₅, resistor R₂₅ and capacitor C₆. The output of the delay circuit is connected to set terminal S of flip-flop circuit FF. Signal Y is applied to the input of NAND circuit NA₂ together with the signal from basting stitch switch SW_(B). The output of the NAND circuit NA₂ is a first input to AND circuit AND₄, for detecting when the motor is to stop. The output of AND circuits AND₁, AND₂ is applied to a second input of AND circuit AND₄ via OR circuit OR. OP₅ is an operational amplifier for detecting a low motor speed prior to stopping. Operational amplifier OP₅ has a non-inverting input connected to the center point of a voltage divider formed by resistors R₂₆ and R₂₇, and has an inverting input connected to terminal F in FIG. 7, which carries the speed signal F graphed in FIG. 9. The output of the operational amplifier OP₅ becomes logically high only when speed signal F becomes sufficiently low so as to cause the motor to stop, and this output is connected to a third input to AND circuit AND₄. The output terminal of AND circuit AND₄ is connected to a reset terminal R of flip-flop circuit FF. A complement side output Q of flip-flop circuit FF is connected to the inverting input of operational amplifier OP₄ via diode D₁₀, and is also connected to one input of NAND circuit NA₃. A true output Q of flip-flop circuit FF is connected to the other input of NAND circuit NA₃ via a delay circuit composed of a resistor R₂₈ and a capacitor C₇. The output of NAND circuit NA₃ issues a base signal to a control transistor Tr₂ via a resistor R₂₉. Transistor Tr₂ has its emitter connected to point A, and has its collector connected to a junction between resistor R₁₅ and light-emitting diode LED₁ via a resistor R₃₀ and light-emitting diode LED₂. Thus, transistor Tr₂ is operated to light light-emitting diodes LED₁ and LED₂ concurrently. Light-emitting diode LED₂ illuminates the phototransistor PTr₄, delivering a gate trigger signal to braking PTr₄, delivering a gate trigger signal to braking thyristor 13. The collector of phototransistor PTr₄ is connected to point X. R₃₁ and R₃₂ designate resistors.

The operation of the combined control circuits shown in FIGS. 5A, 5B and 7 will now be described. When basting stitch switch SW_(B) is opened, the needle stop switch SW_(C) is opened (causing the needle stop position to be the lower dead point), wiper CONT of the control (not shown) is positioned at its lowest position, and when AC source 1 is turned on, the operational amplifiers OP₂ and OP₃ have no output and the operational amplifier OP₁ generates the low speed signal J₁ shown in FIG. 6. But since output Y of operational amplifier OP₃ is logically low, flip-flop circuit FF receives a low set signal at input S and maintains the reset condition which it assumes at power turn on. If AC source 1 is turned on when wiper CONT is positioned at an upper position, output Y of operational amplifier OP₃ becomes logically high. But, capacitor C₆ and inverter IN₅ are logically low, AND circuit AND₃ has logically low output and flip-flop circuit FF remains reset. Thus, flip-flop FF is maintained in a reset condition at the time of turnon of power source 1, even if basting stitch switch SW_(B) and needle stop switch SW_(C) are operated at such time. Hence, complement side output Q is logically high and the inverting input of operational amplifier OP₄ is brought high, its output is brought low, and therefore, said LED₁ is kept off, PTr₁ is not illuminated, and motor 11 remains stopped. When wiper CONT is moved upward in FIGS. 5A, 5B (in case the wiper is at an upper position at the time of power turnon, wiper CONT must be once returned to its lowest position before it is moved up again), voltage at point Y is raised to a logically high level and flip-flop FF receives a signal at set input S for that time required by capacitor C6 to charge via resistor R₂₅ during the rise of the signal Y on the transverse time axis shown in FIG. 11. When the signal Y is logically high NAND circuit NA₂ has a logically low output and the signal at reset terminal R of flip-flop FF is also logically low, causing flip-flop circuit FF to remain set to generate speed signal F via operational amplifier OP₄. F₁ in FIG. 6 shows that when the motor is operating at a low speed, firing of thyristors 10 takes place at the time when signal J₁ crosses speed signal F₁. The phase range shown by oblique lines (in which signal J₁ exceeds signal F₁) varies as speed signal F varies. In this range, the operational amplifier OP₄ has a logically high level output, and light-emitting diode LED₁ thus lights in this range. Hence, the firing phase of thyristors 10 feeds back the motor speed. The respective crossing points of speed signals F₁ -F₄ and their respective speed designating signals J₁ -J₄ show the respective ignition phases of thyristors 10 in dependence upon the speed settings determined by movement of wiper CONT for speed control of the motor. Phototransistor PTr₃ is at off and does not effect AND circuit AND₂ since switches SW_(B), SW_(C) are opened, and AND circuit AND₁ issues a pulse after phototransistor PTr is off. When the controller is released the signal at point Y becomes logically low. Since the operational amplifier OP₅ has a high level output due to rotation of motor 11, the reset input R of flip-flop circuit FF becomes logically high and the flip-flop is reset at the initial conduction of phototransistor PTr₃ after the signal at point Y was low, as is shown in FIG. 11. Thus, the output of operational amplifier OP₄ becomes logically low, and light-emitting diode LED₁ is not energized thereby. However, output Q of flip-flop circuit FF is high, and NAND circuit NA₃ issues a low pulse having a width equal to the charging time of capacitor C₇ and lights light-emitting diodes LED₁ and LED₂ during said pulse. This period has a large width exceeding the period π of power source 1, and light-emitting diode LED₁ instantly short-circuits control thyristors 10 at time t₅ in FIG. 4, and light-emitting diode LED₂ then fires braking thyristor 13 and the field coil 6 is energized while armature 5 is not. The ignition signals for thyristors 10 and 13 instantly disappear, but armature 5 starts to generate electric power as it begins to rotate. As a result, a circulating electric current flows through braking diode 14, field coil 6 and thyristor 13 and the motor is dynamically braked. Motor 11 is stopped after a point which is just before the lower dead point of the needle has been detected (as shown in FIG. 11 for example, after rotation of around 30° ), and the needle of the sewing machine is then set so as to just reach the lower dead point. When the needle stop switch SW_(C) is closed, AND circuit AND₂ is turned on and reset input R of flip-flop circuit FF is pulsed based on the rising current through phototransistor PTr₃ in FIG. 11, and the needle thus stops similarly, but at the upper dead point. When basting stitch switch SW_(B) is closed here, independently of operation of switch SW_(C), and wiper CONT is moved up, flip-flop circuit FF is set by the rising signal Y (shown in FIG. 12) and motor 11 is driven. Since transistor Tr₁ now grounds the output of operational amplifier OP₂, motor 11 is driven at a low speed. When the sewing machine is driven for one rotation and current through phototransistor PT₃ is detected indicating a point just before the upper dead point of the needle, flip-flop circuit FF is reset and the sewing machine is braked as mentioned above and is stopped at the upper dead point of the needle. Subsequently, the machine is rotated once again and is subsequently stopped after one rotation to cause an intermittent stitching operation, such as basting, to be carried out. In FIG. 12, the operation Y of the controller is logically low after the stop signal NA₃ is produced. If the controller is released before the stop signal is produced, braking is not influenced.

FIG. 13 shows another embodiment of the invention, in which elements which are same as those used in FIGS. 3 and 5A, 5B have the same reference numerals. 11a is a braking thyristor connected in series with a braking diode D₁₀ and in parallel with the armature 5. G_(13a) shows its gate terminal. R₃₃ and R₃₄ are resistors, and C₈ is a capacitor. ZD₄ is a diode for operating phototransistor PTt₄. A junction between thyristor 13a and diode D₁₀ is connected to AC source 1 via a diode D₁₁ to supply an energized braking current to a field coil 6 through thyristor 13a. Diode D₁₀ prevents a current from being supplied to the armature 5 through diode D₁₁.

The main difference between the embodiment of FIG. 3 and the embodiment of FIG. 14 is that braking thyristor 13a is ignited by a gate pulse G_(13a) which has a width much narrower than the braking period, and is self-extinguished at time t₈ when rotation N of motor 11a is stopped. If motor speed follows the dotted line in dependence upon motor load, the conductive period of the thyristor 13a will be postponed (as shown by the dotted line) until time t'₈. This will now be explained with reference to FIG. 13. After thyristors 10 are ignited by the ignition signals G₁₀ and motor 11a is rotated, if thyristors 10 are extinguished and thyristor 13a is fired at time t₂ in order to brake motor 11a, the field coil 6 will be energized through diode D₁₁ and thyristor 13a, and armature 5 will generate electric power on the basis of motor rotation at time t₂. As a result, current circulating through diode D₁₀ and thyristor 13a instantly stops the motor 11a. In this instance, the ignition signal G_(13a) of thyristor 13a is logically low, but thyristor 13a is kept conductive by the circulating current until power generation stops.

FIG. 15 shows that cutoff of thyristors 10 need not be coincident with firing of thyristor 13a, as is the case in FIG. 14. If the ignition signal G_(13a) comes slightly earlier than the disappearance of the ignition signal G₁₀, motor braking is initiated on the basis of the ignition signal G₁₃. In this case, current flowing through the thyristors 10 temporarily energizes field coil 6 through thyristor 13a, instead of through armature 5.

In FIG. 13, the braking current flowing through diode D₁₁ is half-wave rectified. An additional diode D₁₁ can be connected as shown to produce a full-wave rectified current so as to increase the braking effect.

In accordance with the invention as shown particularly in FIGS. 5A, 5B and 6, the signals J₁ -J₄ at the non-inverting input of operational amplifier OP₄, (which is controlled by operation of the wiper CONT) start from zero potential at a zero crossover of power source 1, and end near the next crossover as substantially straight lines with different slopes. On the other hand, signals F₁ -F₄ at the inverting input of operational amplifier OP₄ rise in proportion to motor speed. The crossing points of the two inputs of OP₄ can control the ignition angle of the thyristors 10 in a broad range. In the case of this voltage comparison, only a voltage proportional to motor speed is used while voltage of the power source is not. Therefore, speed response is good. Especially during low speed rotation of motor 11, the inclination of signal J₁ is small, causing the ignition phase of thyristors 10 to be broadly varied by any change in motor speed due to the variation of machine load. Thus, the invention provides an excellent control for motor speed and braking.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of sewing machine motor control systems differing from the types described above.

While the invention has been illustrated and described as embodied in a control system, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.
 1. A speed control circuit of a motor used in combination with a sewing machine having a main drive shaft rotated by the motor to operate the stitch forming device of the sewing machine, said speed control circuit including a controller, thyristors (10) for controlling the rotation of the motor in dependence upon the operation of the controller, said speed control circuit further comprising means (OP₁) for producing a constant triangular wave voltage, means (C₂, OP₂) responsive to operation of the controller to change the constant triangular wave voltage into a variable speed-designating voltage (J) in a period of an AC power source, a frequency-voltage converter receiving a frequency representing the rotation speed of the main drive shaft of the sewing machine so as to produce the corresponding voltage (F), a comparator comparing the variable speed-designating voltage (J) and the voltage of said frequency-voltage converter to detect a motor drive phase (C) of the speed-designating voltage (J), and control means (LED₁, PTr₁) activated by the motor drive phase of the speed-designating voltage (J) to ignite the thyristors (10).
 2. The circuit as defined in claim 1, wherein said means to produce a constant triangular wave voltage include a first operational amplifier (OP₁) receiving an AC power source voltage and said means to change the constant triangular wave into a variable speed-designating voltage include a second operational amplifier (OP₂) and a capacitor (C₂), said second operational amplifier being operative to accelerate the charge of the capacitor (C₂).
 3. The circuit as defined in claim 2, wherein said frequency-voltage converter includes a phototransistor (PTr₂), the sewing machine being provided with a screen disc connected to the main shaft and adapted to actuate said phototransistor. 